INTENTO: A cold is a respiratory illness principally affecting the respiratory tract and lungs (although other parts of the body may also be involved). For many people it is accompanied by a general sensation of malaise. [VER a-d]. Often conditions affecting the respiratory tract at a deeper level*, such as bronquitis, begin as a nasal/head cold.
*affecting a deeper/lower level of the respiratory tract?

a. Since nasal catarrh (rhinitis, runny or stuffy nose) is the commonest upper respiratory tract infection, it is generally known as the “common cold”.
b. The term “common cold” is also used to describe a nasal/head cold, the most frequent upper respiratory tract infection.
c. Because a nasal/head cold is the most frequent form of upper respiratory tract infection, it is often referred to as a “common cold”.
d. Because a nasal/head cold is the commonest/most common? form of upper respiratory tract infection, it is often referred to as a “common cold”.

A common cold is a cold which mainly affects both respiratory tract and lungsd (although other parts of the body can also get sick). It also causes discomfort in many people. It is also common to call cold to a head cold, as it is the most frequent of these conditions. In many occasions, more serious illnesses of the respiratory tract, such as the bronchitis, start with a common cold o appear as a consequence of a common cold.

¿En qué punto encuentras que he cambiado el tono del texto original? Aprovechando algunas de tus sugerencias, mi nuevo intento sería:
A common cold is a respiratory illness principally affecting the respiratory tract and lungs (although other parts of the body may also be involved). For many people it is accompanied by a general sensation of malaise. A nasal/head cold is often referred to as a “common cold” because it is the most frequent of these conditions. Often illnesses affecting the respiratory tract at a deeper level, such as bronchitis, begin as a nasal/head cold.